The enhancement of EM fields in the vicinity of metallic nanoparticles and metallic nanostructures can be explained by the phenomenon of localized surface plasmon resonance. The shape and magnitude of associated features measured in the transmission or reflection spectra from these metallic structures depend on the enhanced scattering and absorption of light at specific wavelengths. The details of the extinction cross-section enhancement over a finite wavelength range is affected by several different factors that include the characteristic optical constants and geometry of the nanostructures illuminated by incident light as well as the optical constant of the surrounding matrix phase.
The origin of plasmon resonances are collective oscillations of the conduction band electrons, and they result from the presence of interfaces for nanoparticles and films of a select group of materials which include the noble metals Ag, Cu, and Au. Localized surface plasmons are excited when light is incident on metallic nanoparticles which typically have dimensions smaller than the wavelength of the incident light. At certain characteristic wavelengths, one or more resonant modes are excited in the nanoparticles, leading to a significant enhancement in absorbed and scattered light and a strong increase in the electromagnetic fields in the vicinity of the particles. Localized surface plasmons can be detected as resonance peaks in the absorption and scattering spectra of the metallic nanoparticles. Nanostructures made up of noble metals, such as gold, silver, and copper, are well known to exhibit localized surface plasmon resonance (LSPR) phenomena.
The collective oscillation of the free electrons is also sensitive to changes in the size of the particle. For example, gold nanoparticles embedded in a transparent matrix phase with a real dielectric constant similar to that of SiO2 (ε˜2.25) and average diameters in the range of approximately 5-10 nm, strongly absorb at visible wavelengths with a maximum absorbance near 520 nm. In this particular case, the energy required to excite the surface plasmon lies in the visible region of the spectrum. With increases in the Au particle size, a shift in the peak of the optical absorption to longer wavelengths is observed due to the excitation of higher-order resonant modes. The relative magnitude of the scattering cross-section also increases as compared to the absorption cross-section resulting in particles that strongly scatter light rather than absorb it for particle sizes approaching 100 nm. In addition to being site-dependent, the plasmon resonance band is sensitive to changes in the dielectric properties of the surrounding medium. For transparent matrix media with large dielectric constants, the energy required to collectively excite the electrons is decreased thereby shifting the peak in the extinction cross-section to lower energies and longer wavelengths.
The strong dependence of the optical extinction peak on a number of material dependent parameters provides the nanoparticles with an inherent sensing ability. For visible light, generally only changes in refractive index occurring at distance within about 200 nm of the particle surface result in changes to the optical properties of the nanoparticles. The plasmon resonance behavior of nanoparticles are particularly sensitive to adsorption directly on the particle surface and hence biological sensing based on analyte absorption by nanoparticles and subsequent modifications of the absorbance maximum is currently an area of significant effort.
The changes in the absorbance maxima generated by the localized surface plasmon resonance effect have also been utilized extensively for gas sensing applications in the low and intermediate temperature ranges. A select few researchers in the field have also applied Au incorporated films to optical gas sensing at higher temperatures. However, current technical literature suggests that the fundamental response of technically useful Au/metal oxide composite films requires the selection of a matrix phase that plays an active role in the gas sensing process. Two potential ways that such an active role can be played include (1) a change in the free carrier density of the matrix phase followed by an electronic charge transfer from the matrix to the nanoparticle and (2) a change in the effective dielectric constant of the matrix phase. Both of these effects would result in a modification to the extinction peak of Au nanoparticles associated with the localized surface plasmon resonance effect that could be detected through optical based monitoring techniques. As a result, nanoparticles are generally embedded in catalytically active, reducible, and oxygen conducting matrices such as TiO2 or yttria-stabilized zirconium (YSZ) for high temperature (T>˜500° C.) optical gas sensing. Current technical literature suggests that technologically useful optical responses in this temperature range associated with plasmon absorption peak shifts require the presence of oxygen in the sensing environment and reduction of the matrix phase with associated changes in the oxygen vacancy concentration. For lower temperatures, gold nanoparticles have been embedded in matrices of even more highly reducible oxides such as NiO, WO3, or CuO. The sensing mechanism is again attributed to reduction of the matrix in the bulk or at the surface, and modifications to the real and imaginary parts of the refractive indices of the oxide may play a more significant impact on altering the position and shape of the SPR band. See e.g., Sirinakis et al., “Development and Characterization of Au-YSZ Surface Plasmon Resonance Based Sensing Materials: High Temperature Detection of CO,” J. Phys. Chem. B 110 (2006); and see Ando et al., “Optical CO sensitivity of Au—CuO composite film by use of the plasmon absorption change,” Sensors and Actuators B 96 (2003); and see U.S. Pat. No. 7,864,322 B2 to Carpenter et al. These approaches have the disadvantage of requiring the concurrent presence of O2 as a gaseous constituent in order to affect charge transfer and/or limited stability of the oxide matrix.
Another class of dielectric matrix phases exists which exhibits improved temperature and chemical stability as compared to reducible oxides and which is characterized by a relatively wide bandgap and low oxygen and electronic conductivity commonly referred to in the literature as “inert”. Examples of such inert materials include SiO2, Al2O3, and Si3N4. In this class of matrix materials, direct interactions of the matrix with the ambient atmosphere are mitigated as compared to catalytically active, reducible, and oxygen conducting matrices such as TiO2 and YSZ. It would be advantageous to provide a plasmon-based methodology for high temperature gas sensing based on a gas sensing material comprised of Au-nanoparticles embedded in such an inert matrix, where technologically useful responses are observed despite mitigation of interactions between the matrix material and the ambient gas atmosphere reported to be responsible for the measured sensing response in systems such as Au/YSZ. Direct interactions with oxygen conducting matrices such as YSZ generate a change in effective dielectric constant and/or a change in the concentration of oxygen vacancies with a measurable effect on the localized surface plasmon resonance (LSPR) extinction cross-section. The former impacts the LSPR behavior directly while the latter affects it through changes in the density of electrons (n-type oxides) or holes (p-type oxides) in the matrix phase, followed by charge transfer between the nanoparticle and the matrix. It would also be advantageous to provide a methodology that does not require the presence of O2 within the sensing environment.
Preferably, such a methodology would generate useful responses due to a direct action between the nanoparticle and ambient chemical species, particularly at temperature in excess of approximately 500° C. Currently, plasmonic responses to such direct interactions have been limited to a gold nanoparticle/silicon substrate material exposed to atomic H at room temperature. See Giangregorio et al., “Insight into Gold Nanoparticle-Hydrogen Interaction: A Way To Tailor Nanoparticle Surface Charge and Self-Assembled Monolayer Chemisorption,” J. Phys. Chem. C 115 (2011). Further, investigations on the adsorption of diatomic H2 on gold nanoparticles has been limited to temperatures of around 250° C., significantly below the 500° C. or greater temperatures desired for certain operations including, but not limited to, power generation technologies utilizing fossil fuels including coal gasification, solid oxide fuel cells, gas turbines, and advanced combustion systems. See e.g., Bus et al., “Hydrogen Chemisorption on Al2O3-Supported Gold Catalysts,” J. Phys. Chem. B 109 (2005). Other investigators have suggested that direct adsorption of H2 on Au in the Au/YSZ system could be responsible for observations in O2-free atmospheres at temperatures approaching 500° C., but technologically useful responses were not observed and it was indicated that measured responses could also be explained through donation of electrons to the matrix phase by atomic H+ followed by charge transfer between the matrix and Au nanoparticles leading to plasmon resonance shifts. See e.g. Joy et al., “Plasmonic Based Kinetic Analysis of Hydrogen Reactions within Au-YSZ Nanocomposites,” J. Phys. Chem. C 115 (2011). It would be advantageous if a methodology were provided whereby nanoparticles dispersed on, beneath, or embedded within a matrix could be utilized for a plasmon-based detection of gas species generated through a direct interaction between the nanoparticles and the ambient gas species, and additionally advantageous if the methodology were effective in gas streams at temperatures in excess of approximately 500° C., such that they were relevant for a number of fossil fuel based energy production applications.
Disclosed here is a method for gas sensing utilizing shifts in a plasmon resonance peak position generated by a gas sensing material. In an embodiment, the gas is at a temperature greater than about 500° C. The gas sensing material is comprised of a plurality of gold nanoparticles dispersed in a wide bandgap matrix with a sufficiently low oxygen ion conductivity at the temperatures and gas atmospheres of interest that it is classified as “inert”. The method disclosed offers significant advantage over materials typically utilized for plasmon-based high temperature sensing such as yttria-stabilized zirconia (YSZ) or TiO2, including enhanced thermal and chemical stability and increased stability of nanoparticle diameter, among other advantages. In addition, several candidates for inert matrix materials (e.g. SiO2, Al2O3, MgF2 doped SiO2, mixed SiO2/Al2O3) exhibit relatively low values of refractive indices for fully densified films ranging from less than ˜1.5 to greater than ˜1.7. In contrast, such low values of effective refractive index cannot be obtained in fully dense films of reducible or high oxygen conducting oxides such as TiO2 and YSZ. This property is advantageous as it enables integration of nanocomposite films directly with optical fiber based sensors as a gas sensitive cladding layer in an evanescent wave absorption spectroscopy based sensing configuration, while maintaining the conditions necessary for waveguiding in the low refractive index core material. Typical core materials for such applications include SiO2 (refractive index ˜1.5) for silica-based optical fibers and Al2O3 (refractive index ˜1.7) for sapphire based optical fibers.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.